Functions and requirements of ingredients of cement concrete
The importance of the ingredients should be known before there are used in cement concrete.
Cement is binding material in the cement concrete. This concrete is used for different engineering works where strength and durability are of Prime importance.
Functions of cement
- It fills up voids existing in the fine aggregate and makes the concrete impermeable.
- It provides strength to concrete on setting and hardening.
- It binds the aggregate into a solid mass by virtue of its setting and hardening properties when mixed with water.
Requirements: Good cement should satisfy all the requirements as per I.S. specifications
In the cement concrete, to provide good quality of concrete aggregate is used in two size groups:
· Fine aggregate (sand) particle size less than 4.75mm
· Coarse aggregate – Particle size more than 4.75mm
Fine aggregate (sand)
Sand consists of small angular or rounded grains of silica. Sand is commonly used as the fine aggregate in cement concrete. Both natural and artificial sands are used for this purpose.
Functions of sand:
1. It fills the voids existing in the coarse aggregate.
2. It reduces shrinkage and cracking of concrete.
3. By varying the proportion of sand concrete can be prepared economically for any required strength
4. It helps in hardening of cement by allowing the water through its voids.
5. To form hard mass of silicates as it is believed that some chemical reaction takes place between silica of sand and constituents of cement.
1. Fine aggregate should consist of coarse angular sharp and hard grains.
2. It must be free from coatings of clay and silt.
3. It should not contain any organic matter.
4. It should be free from hygroscopic salt.
5. It should be strong and durable and chemical inert.
6. The size of sand grains should pass through 4.75mm IS sieve and should be entirely retained on 75 micron IS sieve.
· Coarse aggregate makes solid and hard mass of concrete with cement and sand.
· It increases the crushing strength of concrete.
· It reduces the cost of concrete, since it occupies major volume.
Requirements: Coarse aggregate used may be crushed stone, gravel and broken bricks.
Crushed stone: It is an excellent coarse aggregate provided. It is obtained by crushing granite, sandstone and close grained limestone.
Crushed granite chips are commonly and advantageously used in reinforced cement concrete.
Broken bricks well burnt and over burnt bricks are broken into suitable size and used as aggregate. It should be well watered before its use. Broken bricks are used as aggregate for concrete in foundations and under floors.
But generally crushed stone is only used as coarse aggregate.
The water is used in concrete plays an important part in the mixing, laying compaction setting and hardening of concrete. The strength of concrete directly depends on the quantity and quality of water is used in the mix.
1. Water is only the ingredient that reacts chemically with cement and thus setting and hardening takes place.
2. Water acts as a lubricant for the aggregate and makes the concrete workable.
3. It facilitates the spreading of cement over the fine aggregate.